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The authors do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.

For the second time in four years, the Glasgow School of Art has been devastated by fire. The art school’s historic Mackintosh building, which was well on the way to being restored after the fire of 2014, has been extensively damaged in the blaze, badly affecting every floor.

More than 120 firefighters and 20 fire engines were at the scene late on Friday June 15, but were unable to stop the fire spreading to the Campus nightclub and O₂ ABC music venue on Sauchiehall Street. The ABC has also been badly damaged, with a major part of the roof collapsing.

Iain Bushell, the deputy chief fire officer on the scene, called the fire an “extremely challenging and complex incident”. Thankfully nobody was injured. Yet well into Saturday afternoon, firefighters were still working hard to put the blaze out.

While the city’s residents come to terms with yet another dreadful fire in the city centre – an area only a couple of blocks away is still cordoned off following a major nightclub fire in March – here are our early thoughts about the causes and implications:

1. The cause of the fire

At this stage there are more questions than answers. It could have been caused by a small fire that burned for a substantial length of time and then accelerated – or it could have grown much more rapidly. Either way, there was a fully developed fire when the Fire Service arrived soon after the alarm was raised in the late evening.

The undiscovered, slow burning fire seems less likely. The upper floors and roof appear to have been well ablaze from the first images reported, which suggests the fire started on the upper levels and burned down through the building.

When a building is under construction – or in this case reonstruction – it is much more vulnerable to fire. It can mean more timber is exposed, as well as there being other openings in the structure that can allow a fire to spread unchecked.

Having said that, a typical cause of ignition on construction sites is “hot work” involving flames. Yet our understanding is that there was no such work taking place, and no workpeople actually on site.

Another common cause of fires is old faulty wiring. In 2002, for instance, a fire in the Gilded Balloon building in Edinburgh’s Old Town started from a faulty fuse box. It took 52 hours to fully extinguish and engulfed 11 buildings. Yet in the case of the Mackintosh building, faulty wiring is unlikely to have been the cause, given the late stage of the refurbishment.

2. How it spread

While it is not certain from the video footage and photos, the collapse of the roof of the O₂ ABC appears to have been caused by fire inside the building as opposed to fire penetrating the roof from the McIntosh building. This might raise issues about the fire separation between the two buildings.

When such a close group of buildings is erected today, there are strict rules about separation in the building regulations. But these cannot apply to historic buildings that have been adapted over many decades. Fires in historic buildings are not uncommon – see here for all those that were damaged in the UK last year, for example.

The restoration work to the Mackintosh building was well underway from the fire four years ago – apparently around 80% completed. It was due to reopen next year with a final bill estimated at between £20m and £35m. Investigators will want to know about the specialist work that was being done, what materials were being used and which were on site.

Once you have high enough temperatures, of course, most things will start to burn. The Fire Service appear to have had a very challenging job just to limit the spread – let alone put the fire out altogether.

3. What happens next?

The damage at the Mackintosh building appears overwhelming, and much worse than in 2014, when recovered materials were painstakingly assessed and used in the refurbishment wherever possible. It seems questionable whether anything will be salvaged in the same way after this fire.

It remains to be seen if it will be possible to retain a facade from the current building. If not, damaged buildings have been taken down almost stone by stone in the past and rebuilt with a new, internal frame. This sort of project would cost a great deal more than the current refurbishment.